1. Field of the Invention
The present invention is broadly concerned with an improved system for selectively detecting low molecular weight gases such as ammonia and carbon monoxide in the presence of a variety of contaminant gases. More particularly, it is concerned with a gas detection system having an organic molecular sieve filter, a semiconductor sensor, an electronic control system, and an alarm. The system of the invention may be used to good effect to continuously monitor for selected low molecular weight gases in hot environments subject to periodic contamination by alcohols, methane, acetic acid, and other nonselected gases.
2. Description of the Related Art
Related art gas detection systems for small molecular weight gases such as ammonia and carbon monoxide lack specificity in that they also recognize a variety of contaminant gases. Such gas detection systems often employ solid state semiconductor gas sensors, broad spectrum devices which are extremely durable and particularly well suited for use in hot environments such as machine rooms. Despite this lack of specificity, in many applications systems employing semiconductor sensors are preferred over those which employ electrochemical sensors, which are more expensive, short lived and may be depleted by a single large gas leak. Such solid state systems are also often preferred over systems employing infrared or ultraviolet light sensors, which are expensive and require complex apparatus to monitor a large area.
For example, ammonia sensors are employed in the food industry, where processing and distributing environments typically include commercial refrigeration systems which employ compressed ammonia gas. Gas detection systems which are currently available to detect leakage of such ammonia refrigerant include sensors which may be tripped by nonammonia-containing gases. Such contaminant gases may emanate from a variety of sources including cleaning chemicals, such as chlorine, ozone, alcohols, and solvents. Alcohol in particular is present in many commercial environments, and heretofore gas detection systems have often been unable to distinguish ammonia gas from alcohols. Other sources of contaminant gases include product off-gassing such as ethylene, and methane gases emanating from food wrappings; use of internal combustion engines, which produce carbon monoxide and unburned hydrocarbons; and air borne, non-gaseous particulate matter such as flour, dust and grease.
In addition, variations in temperature, moisture levels, and air composition, such as low oxygen levels occurring in certain fruit storage areas, may affect the functioning of available gas sensing devices.
Carbon monoxide sensors are employed in both domestic and commercial environments where combustion used to generate heat and air conditioning may also generate toxic gases. Current carbon monoxide gas detection systems may include broad spectrum sensors which may be tripped by other large or small molecular weight gases, a characteristic which increases with the age of the sensor. Maintenance of such systems is expensive and time-consuming, because aging sensors must be replaced at intervals of about every two years in order to compensate for their increased sensitivity. Failure to conduct such frequent maintenance results in false alarms, which may encourage the user to incapacitate the device entirely. In addition, false alarms create the possibility that a true alarm will be ignored.
Previous attempts to employ various kinds of filters in such heated solid state gas sensor devices have been unsuccessful because such filters have been fouled by water vapor present in the ambient atmosphere and negatively affected by the high temperatures caused by the heater component of the device.
Accordingly, there is a need for an improved gas detection system which permits only selected low molecular weight gases to reach a solid state semiconductor sensor, and prevents nonselected, contaminant gases from reaching the sensor.
U.S. Pat. No. 5,296,196 discloses a device for detecting large molecular weight hydrocarbons. The device includes a semiconductor sensor having a zeolite inorganic molecular sieve filter. The pores of the zeolite are doped with a catalyst metal. Small molecular weight hydrocarbons are caught in the pores and oxidized by the metal into water and carbon dioxide, so that only large molecular weight hydrocarbons pass through the filter to the sensor.
U.S. Pat. No. 5,143,696 discloses a device for detecting small molecular weight gases. The device includes an electrochemical sensor having a capacitor coated with a layer of a zeolite inorganic molecular sieve serving as a dielectric. The zeolite is doped with metals for oxidizing or adsorbing of molecules which diffuse into the zeolite pores.